1. Field of the Invention
The present invention relates to a high-frequency power amplifier for use in mobile communication devices and the like, and a communication apparatus having such a high-frequency power amplifier.
2. Description of the Background Art
A high-frequency power amplifier for use in mobile phone terminals of GSM (Global System for Mobile Communications) system and the like, controls the magnitude of the output power based on the distance thereof from the base station. FIG. 17 is a circuit diagram illustrating a high-frequency power amplifier 10 (hereinafter referred to as the “amplifier 10”) which is an exemplary conventional high-frequency power amplifier. The amplifier 10 includes a bipolar transistor 11, a base bias circuit 12, a voltage regulator 14, an RF choke inductor 17, an input matching circuit 15, and an output matching circuit 16.
The RF choke inductor 17 and the voltage regulator 14 are sequentially connected in this order to a collector terminal of the bipolar transistor 11. A power control signal Vctrl for adjusting the signal amplification level of the bipolar transistor 11 is inputted to the voltage regulator 14. An emitter terminal of the bipolar transistor 11 is grounded. A high-frequency input signal RFin is inputted to a base terminal of the bipolar transistor 11 through the input matching circuit 15. The base bias circuit 12 adjusts the magnitude of a base voltage Vbb. The high-frequency input signal RFin is amplified by the bipolar transistor 11 and then outputted as a high-frequency output signal RFout from the collector terminal of the bipolar transistor 11 through the output matching circuit 16.
In the case where the input power to the base terminal is sufficiently high and the bipolar transistor 11 operates as a saturation amplifier, an output power proportional to almost the square of collector voltage Vcc can be obtained with respect to a given input power. Note that although, for simplification, FIG. 17 shows a single-stage amplifier 10, normally, an amplifier 10 in which bipolar transistors 11 are cascade-connected in two or three stages is used.
In communication systems such as EDGE (Enhanced Data Rate for GSM Evolution), a technique called a polar modulation is used. In this case, as a power control signal Vctrl, an amplitude modulation signal is inputted to the collector terminal, and thereby amplitude modulation is performed, which is described in International Publication No. WO-02/101944, for example.
The output power of the amplifier 10 changes with the device temperature of the bipolar transistor 11 even if the collector voltage Vcc and the base voltage Vbb are constant. Therefore, conventionally, by adjusting the base voltage Vbb in the base bias circuit 12 based on the temperature, temperature-dependent variations in output power are minimized, which is described in, for example, Japanese Examined Patent Publication No. 8-28617 and Japanese Laid-Open Patent Publications No. 2001-176982 and No. 2002-9558.
However, even if temperature compensation is thus performed on the base voltage Vbb, in the case of low output where the collector voltage Vcc is low, more specifically, in the case where the bipolar transistor is used in a saturation region, output voltage fluctuations caused by changes in temperature cannot be sufficiently minimized. FIG. 18 is a diagram showing the relationship between the collector voltage Vcc and the output power of a conventional high-frequency power amplifier for the GSM system for the case where the device temperature is 90° C., 30° C., and −30° C. It can be seen from FIG. 18 that in the region where the collector voltage Vcc is less than 0.6 V, the lower the collector voltage Vcc, the greater the change in output power with the temperature.
A high-frequency power amplifier for use in GSM system mobile phones is used in a region where the output power ranges from 5 to 35 dBm shown in FIG. 18. That is, the high-frequency power amplifier is used also in a region where the output power variations with the temperature are great. In EDGE system mobile phones, the amplitude modulation signal to be inputted to the collector terminal is used in a region where the output power ranges from −8 to 32 dBm shown in FIG. 18. That is, the EDGE system mobile phone is used in a region where the output power variations with the temperature are more significant than that of the GSM system mobile phone. In addition, in the EDGE system using polar modulation, an amplitude error is directly related to degradation of modulation accuracy, and therefore, the EDGE system requires better output stability than the GSM system. The polar modulation is a technique which can be applied also to a W-CDMA (Wideband CDMA) system in the future. The W-CDMA system needs to be controlled at a lower output power than that for the EDGE system. Therefore, a technique is demanded for minimizing output power fluctuations caused by changes in temperature, at low output power where the bipolar transistor needs to be controlled in the saturation region.